An AC generator is comprised of two major parts, the stator winding and the rotor (field) winding. The stator winding is housed in the body of the generator and is physically connected to the system. The rotor winding is wrapped around a shaft which, when driven by a prime mover such as a steam turbine, rotates within the stator. By passing current through the rotor winding as the shaft is rotated by the prime mover, voltage is produced on the stator winding. As power is increased to the prime mover (e.g. by increasing steam pressure to a steam driven turbine), more current flows through the stator winding and thus more power (watts), is delivered to the system. By increasing the current to the rotor winding, the generator produces more `vars` which, in effect, can raise the system voltage. Conversely, by decreasing the current in the rotor winding, vars are absorbed by the generator, effectively lowering system voltage.
Obviously, there is a limit to the amount of current that can flow through the stator and rotor windings which, in turn, imposes a limit on the amount of watts and vars that the generator can deliver to the system. There is also a minimum value of current that must flow in the rotor field to maintain generator stability, and this imposes a limit on the amount of vars that the generator can absorb from the system. These watt and var limitations can be represented graphically as the generator's capability curve. As long as the operating point of the generator (i.e. the amount of watts and vars flowing out of or into the generator) is inside the capability curve, the generator will be operating within its safe limits.
On large units, hydrogen gas is circulated through the generator to cool the stator and rotor windings, thus allowing more current flow. Therefore, increasing the pressure of the hydrogen gas in the generator increases its capacity.
Devices have been designed to indicate the operating point of a generator with respect to its capability curve. Such devices are useful in maintaining the operation of a generator within safe limits, and also allow the generator to be operated at its full capacity, providing maximum power to the electrical system. Devices currently in use include an electromechanical movement having two markers movable to positions indicative of watt and var flow, respectively, which cross at the operating point against a fixed background display of the generator's capability curve.